In an electrographic image duplicating apparatus, the images on an original document are optically scanned by illumination with a wide beam of light so that the light incident on the document is modulated depending on the patterns of the images exposed to light. The light reflected from the document and thus carrying information representative of the images scanned is projected onto the photosensitive peripheral surface of a rotating drum, as well known in the art. The images on an original document may be denser in one distinct area than in another of the document. For this reason, the intensity of light (herein referred to as illumination value) with which the document is to be illuminated is regulated upon detection of the image density of the document. The images on the document can thus be reproduced with uniform density distribution without respect to the density distribution of the original images.
An electrographic image duplicating apparatus is therefore equipped with an automatic exposure control system which is operative to reduce the illumination value in response to document areas with low density so that the images reproduced from such areas are sufficiently clear and legible on the resultant output of duplication. In response to a document area with denser images contiguous to a background area denser than white, the automatic exposure control system operates to increase the illumination value so that the background area may not become "foggy" on the resultant output. Automatic exposure control systems to achieve these functions are largely broken down to two major categories, one being of the real-time automatic exposure control type and the other being of the premonitored automatic exposure control type. Systems of the real-time automatic exposure control type are used more frequently in low- to medium-speed image duplicating apparatus while those of the pre-monitored automatic exposure control type find typical applications in high-speed image duplicating apparatus.
A real-time automatic exposure control system, an example of which is disclosed in Japanese Provisional Patent Publication (Kokai) No. 57-124374, uses a slot through which to detect the quantity of the light reflected from a row area of the document to be duplicated. The quantity of light thus detected is converted into a corresponding electric signal to regulate the illumination value, viz., the intensity of the light to be emitted from an exposure lamp for the optical scanning of the document. A photodiode is typically used as the photoelectric transducer to detect the quantity of the light reflected from the document.
The illumination value for the exposure of the images on a document is thus regulated on a real-time basis through detection of the quantity of light reflected from the document being scanned. In this real-time automatic exposure control system, however, a problem is encountered in that the exposure lamp used for the irradiation of the document inevitably has a considerable amount of time lag in responding to the control signal produced upon detection of the reflected light. Tests have revealed that such a time lag is of the order of tens of milliseconds in contrast to the response time of the order of microseconds of the photodiode used as the photoelectric detector.
FIG. 1A of the drawings shows an example of a document 20 having distinct black row areas 22 contiguous to less dense areas 24 bearing distributed image patterns represented by rows of letters "A" and "B". When such a document 20 is optically scanned in the direction of arrow S in an image duplicating apparatus equipped with a real-time automatic exposure control system, the rows of images immediately subsequent to the black row areas 22 may be blanked out in the resultant duplication output 20' as indicated at 26 in FIG. 1B. Such blankout has occurred because of the fact that the exposure lamp can not respond to the shifts from the signal resulting from the light reflected from the black row areas 22 to the signal resulting from the light reflected from the adjacent image-bearing areas 24. The rows of the image-bearing areas 24 directly contiguous to the distinct black row areas 22 are irradiated with an illumination value which has been increased in response to the black row areas 22 and fail to be reproduced due to the large amount of light incident on these particular rows. This problem is the more pronounced where original documents have even higher contrasting distinct image areas and the scanning speeds become higher.
On the other hand, a pre-monitored automatic exposure control system operates to preliminarily scan the whole area of an original document to monitor the total density distribution of the document and determine the illumination value to be used for the real scanning of the document. Two scanning cycles are thus used for the duplication of a document, one for the preliminary monitoring of the density distribution and the other for the copying of the document. An example of such a pre-monitored automatic exposure control system is disclosed in Japanese Provisional Patent Publication (Kokai) No. 54-002134. In this pre-monitored automatic exposure control system, the illumination value to be used for the real scanning of the document to be duplicated is determined on the basis of either the average value or the maximum or minimum value of the density detected of the document. The illumination value thus determined of an original document is maintained valid throughout the scanning cycle for the duplication of the particular document and is known to be advantageous especially for high-speed copying. Such an illumination value is however inadequate for properly reproducing a document having distinct image-bearing areas with contrastingly different density levels such as, for example, a document having an area bearing rows of letters and a darker area bearing a photograph.
A pre-monitored automatic exposure control further has a drawback in that the system uses the preliminary scanning of the document and for this reason necessitates an added amount of time for a step which per se does not lend itself to yielding an actual duplication output. Such an added amount of time required for copying operation may as a matter of fact cause the user to have the impression that a pre-monitored automatic exposure control mode or system calls for a prolonged period of time for copying. Such a circumstance will not be very serious for a user who makes a practice of producing a number of copies on a high-speed machine, but would develop into a problem for a user who ordinarily produces a few copies on a low-to medium-speed machine.